1. Field of the Invention
The present invention relates to a wireless acoustic stimulation system for stimulating biological tissue and, in particular, for a receiver-stimulator that converts an acoustic field into electrical power at high energy conversion efficiency to deliver stimulation energy to tissue. The receiver-stimulator has highly isotropic performance based on the mechanical and electrical arrangement of multiple acoustic power harvesting elements in the receiver-stimulator unit.
2. Description of the Background Art
Stimulation of cardiac tissue using acoustic energy based systems comprising a controller-transmitter and one or more implanted receiver-stimulator devices has recently been proposed by the inventors of this patent application and described in detail, for example in published US Application Publication No. 2006/0136004. The controller-transmitter transmits acoustic energy by producing an acoustic field that is transmitted over time. The acoustic field is a propagating acoustic wave defined by its direction and its intensity (i.e., its power per unit area, typically expressed as Watts/meter2). The acoustic field varies and attenuates as it propagates through the body due to absorption, refraction, and reflection. To minimize losses, the controller-transmitter focuses, or attempts to maximize, the acoustic field on the receiver-stimulator. In turn, the receiver-stimulator maximizes harvesting and converting of the acoustic field impinging upon it into electrical power delivered over time to the tissue to stimulate the tissue (stimulation energy). In general, this receiver-stimulator is a specialized transducer, that is, a device that converts acoustic power to electrical power. In another perspective the receiver-stimulator uses the converted power as a tissue stimulator that delivers electrical energy to cardiac or other tissue through tissue stimulation electrodes. The controller-transmitter may be applied externally on the body, but will usually be implanted in the body, requiring that the controller-transmitter have a reasonable size, similar to that of implantable pacemakers, and that the controller-transmitter be capable of operating from batteries for a lengthy period, typically three or more years. The relatively small size and relatively long operational period make it desirable that the receiver-stimulators harvest as much of the acoustic field transmitted by the controller-transmitter as possible. Furthermore, it is desirable to maximize the isotropy of the receiver-stimulator, whereby the electrical output power delivered to the tissue is constant or nearly constant as the receiver-stimulator's orientation is varied relative to the propagation direction of the acoustic field transmitted by the controller-transmitter, as this orientation is not always predictable.
Piezoelectric components, i.e., piezoelectric transducers, are typically used in acoustic applications to convert mechanical vibrations, such as in an acoustic field, to electrical power. They can also be used in the reverse to convert electrical power into a mechanical, vibrational wave, e.g., an acoustic wave. Coupling of mechanical vibrations in an acoustic field to piezoelectric transducers is an important consideration. The mechanical structure, or portions of the mechanical structure surrounding a piezoelectric component, which is exposed to the acoustic field determines the aperture, or surface, for coupling the acoustic field into the piezoelectric component. Generally, there is a tradeoff between aperture size/isotropy and the electrical power produced by an associated piezoelectric component. On the one hand a large aperture is desired to collect more acoustic power (and can then convert it to more electrical power). However, this comes at the expense of isotropy. The larger an aperture relative to the wavelength of the acoustic field it is placed in, the less isotropic it becomes. Therefore, a receiver-stimulator consisting of a single piezoelectric component is limited in its ability to produce high electrical output and exhibit high isotropy. It can either produce high electrical output power or high isotropy, but not both.
The piezoelectric components produce AC electrical power which is not optimized for tissue stimulation. A rectifier component is used to convert this electrical power to an electrical output which can be configured to effectively stimulate the tissue (e.g., into a DC output but other output waveforms are also effective). Furthermore, the AC electrical power produced by separate piezoelectric components can be out of phase, making it difficult to combine these outputs directly without loss of power. Rectifying these outputs prior to combining them reduces this power loss. Therefore, it would be advantageous to have one rectifier associated with each piezoelectric component. Furthermore, we can view the combination of a piezoelectric component, its associated aperture and rectifier as a single harvesting element that is capable of producing electrical power when placed in an acoustic field. The receiver-stimulator is then a collection of multiple harvesting elements whose electrical outputs are combined to deliver an electrical output to tissue in order to stimulate the tissue.
Once constructed, it is important to consider the assessment of the efficiency and isotropy of the entire receiver-stimulator rather than the individual harvesting elements. The Effective Area of the receiver-stimulator can be defined in terms of the electrical output power delivered to the tissue divided by the acoustic intensity (power/(power/meter2)). Efficiency is then a measure of the Effective Area divided by the physical cross-sectional area of the receiver-stimulator that is exposed to an acoustic field. The Highest Efficiency then would be when the Effective Area approximates the physical cross-sectional area of the receiver-stimulator. To associate efficiency with isotropy, the aggregate performance over all possible orientations of the receiver-stimulator to the acoustic field must be considered. It would be desirable to have a receiver-stimulator that has high efficiency and a high degree of isotropy.
It would be desirable to provide implantable receiver-stimulator devices which are able to efficiently harvest power from an acoustic field transmitted from implanted or external acoustic transmitters and convert the acoustic power into stimulating electrical energy in an efficient manner. It would be particularly desirable if the receiver-stimulators could operate with a high degree of isotropy, where the electrical output power delivered to the tissue is constant or nearly constant as the receiver-stimulator's orientation is varied relative to the propagation direction of the acoustic field transmitted by the controller-transmitter, irrespective of whether the individual harvesting elements in the receiver-stimulator are themselves considered to be isotropic or non-isotropic. At least some of these objectives will be met by the inventions described hereinafter.
The following patents and patent publications describe various implantable transducers capable of converting applied acoustic waves into an electrical output: U.S. Pat. Nos. 3,659,615; 3,735,756; 5,193,539; 6,140,740; 6,504,286; 6,654,638; 6,628,989; and 6,764,446; U.S. Patent Application Publications 2002/0077673; 2004/0172083; and 2004/0204744; and published German application DE 4330680. U.S. Pat. No. 6,504,286 by Porat et al. describes a miniature piezoelectric transducer for providing maximal electric output when impinged by external acoustic waves in the low frequency range. The patent discloses various techniques including the aggregate mechanical structure of the device being omni-directional, changing the mechanical impedance of the piezoelectric layer, etc. As mentioned earlier, it would be desirable to have the receiver-stimulator itself be virtually isotropic rather than rely on the isotropy characteristics of the individual harvesting elements.